A subtrope of Knowledge Broker and Collector of the Strange, the Keeper of Forbidden Knowledge is a being, organization, or even an Eldritch Abomination that contains and sometimes trades information considered either dangerous, blasphemous, epically ground breaking, or any combination of the three. Naturally however, this knowledge comes at a price that makes the trope akin to the Deal with the Devil, and generally leads to disaster for any party involved and then some.
Regarding what form the forbidden knowledge takes, a Tome of Eldritch Lore is almost expected to be the exception. However, sometimes the knowledge can come in the form of a living creature, memory, or (in a more modern setting) a large selection of files on a super computer. Whatever form the knowledge comes in, it usually (but not always) feels like Sealed Evil in a Can.
The Keeper itself is typically something ancient, menacing, or powerful enough that no one can simply ask for the information it has without paying a price. Sometimes the Keeper is a title passed down by an order however, and this is generally the oldest or most intelligent of the conspiracy's agents.
Finally, the Keeper usually lives in an isolated or hazardous area of the society or region it lives in, a better way to test would-be knowledge seekers of their worth and capability to become puppets for the Keeper's whims, sometimes turning said keeper into a Chess Master.
The general moral of stories that revolve around this trope is usually that some things are best left not being known, or that with knowledge comes power, and with power that holds no compassion comes the damning corruption most Keepers of Forbidden Knowledge hope to inspire.
Compare the much more benevolent Ancient Keeper.
- A Certain Magical Index: The eponymous Index is a girl with perfect memory who memorized 103,000 grimoires and therefore has an almost complete knowledge of all magic, including the forbidden stuff. She has some kind of immunity to the usual mind destroying effects reading those grimoires has, which is why the organization she belongs to had her read them: easy access to all the knowledge they wanted. Unusually for the trope, she's a 14 year old girl rather than some ancient creature.
- Fall of Cthulhu introduces the Harlot, an obscene Eldritch horror who parodies a burlesque madame. She offers incredible and unspeakable knowledge, but in return the student becomes her property kept inside a wicker box. The Harlot is depicted as a rival and opponent of Nyarlathotep.
- Cthulhu Mythos: Nyarlathotep is characterized like this in most of his original appearances. Both in "The Haunter of the Dark" and "The Dreams in the Witch House" he is a gatekeeper to secrets beyond human ken, and must be appeased with human sacrifices in order to relinquish some of this knowledge, and in The Dream-Quest of Unknown Kadath he protects the Gods of the Earth from any mortal molestation. In his original appearance in the prose-poem "Nyarlathotep" he instead grants humanity information in abundance as a figure of science, leading to the entire world Going Mad From The Revelation.
- Doctor Who New Adventures: The Library of St John the Beheaded collects and stores forbidden texts (both in the sense of banned-by-the-mundane-authorities and in the sense of Knowledge For Which The World Is Not Ready). It's definitely got the isolated or hazardous location part of the trope down pat; on Earth, it was hidden away in The City Narrows, and in The Future it will be located on an asteroid as part of the Braxiatel Collection, which stores texts for which many worlds are not ready. In principle, it's an aversion of the usual some-things-man-should-not-know corollary: the founder believed that all knowledge is useful if handled carefully, and potential researchers are vetted very carefully before being offered access to the collection. (The Doctor, of course, holds the first ticket the Library ever issued.) In practice, though, pretty much every time it's appeared in a story it's because somebody's found a way to use information from the Library to cause trouble.
- Zeus Is Dead: A Monstrously Inconvenient Adventure has the Moirae of classical mythology (a.k.a. the Fates), though they're not evil so much as they are very, very detached. And strict. Apollo manages to get some information out of them, but only at the cost of his own skill with prophecy.
- Blake's 7. An Invoked Trope with the Clonemasters. The Terran Federation feared that cloning would lead to genetic stagnation, or be used as a weapon against them. Aware however that knowledge can't be unlearned, the Clonemasters were created as a pseudo-religious organisation to restrict the knowledge, yet have it available as a Godzilla Threshold against an enemy with a Clone Army. To ensure the maintenance of their own ethical standards, the Clonemasters clone themselves and indoctrinate the clones in their own belief system.
- Tren Krom was a kind of artificial gelatinous Eldritch Abomination that temporarily controlled the Matoran Universe (actually a space-faring Humongous Mecha named Mata Nui) before Mata Nui's consciousness was activated. He kept the forces of nature in check and was banished to a faraway island for his services, being one of the few inhabitants to know the Matoran Universe's secret and purpose. Makuta Teridax's Evil Plan hinged on sending his brother Mutran to find Tren Krom and essentially allow himself to get mind raped. Mutran lost his sanity in the process but became the universe's most knowledgeable Mad Scientist. Teridax used the info Mutran got from Tren Krom to facilitate his takeover of the universe, and also learned tricks like controlling natural forces via Mind over Matter.
- Unknown to them, the Makuta species was supposed to know the same secret, having been encoded into their minds before they were born. It just so happened that a glitch in their creation made them forget, hence why they needed to consult Tren Krom and used his knowledge for evil. The unborn Makuta in a vat of liquid Antidermis were still aware of Mata Nui's workings and the goal of his space-mission, but without a body they had no way of reaching out to their living brethren until it was too late to stop Teridax's plan.
- Destroy the Godmodder: Several players have claimed to be or actually are this. The_Serpent is/was the only player with knowledge of the psi-godmodder artifacts.
- Dungeons & Dragons: The ancient demon lord Dagon lives within the Abyss, but holds a vast array of information due to his age and intellect, to the point he's become the shadow ruler behind the Prince of Demons' (Demogorgon) entire war stratagem and survival.
- Exalted: Orabilis, the End of All Wisdom, is a demon lord who spends his time building libraries and killing demons who learn things they're not supposed to.
- Pathfinder: Elder sphinxes hoard great stores of magical and occult lore alongside considerable historical knowledge, and are often sought out for the wisdom and information they possess. If someone manages to offer up knowledge that the sphinx does not already possess, it may even agree to share what was sought.
- Diablo IV introduces the Tree of Whispers. The many heads hanging from its branches still retain all of the knowledge they gained in life. The Tree will offer its knowledge to anyone who asks...in exchange for their own head to be added to the Tree's branches after they die.
- In The Elder Scrolls series, this is within the sphere of Hermaeus Mora, the Daedric Prince of Knowledge. Mora specializes in forbidden knowledge, and is the only Daedric Prince who doesn't even bother assuming a humanoid form when dealing with mortals, instead preferring a truly Eldritch Abomination form of a mass of eyes, tentacles, and claws. Mora himself claims to be one of the oldest Daedric Princes, a Time Abyss class of deities, forming out of detritus concepts ejected from reality. Mora's preferred method of seducing mortal servants is to bribe them with gifts of power and knowledge. He also tends to give them absolute freedom, trusting that the lure of the gifts he offers will keep them in his service. His realm, Apocrypha, an endless library containing all knowledge in the form of tomes and very much an Eldritch Location. The sky is an illuminating green in color and it is covered by a sea of roiling acidic waters. Some areas of Apocrypha are consumed by a darkness which kills any mortal who tries to enter it. The realm is haunted by the ghosts of mortals forever searching for knowledge, while it is maintained by Mora's servants, the Seekers and Lurkers. Mora's most famous associated artifact is the Oghma Infinium, a Great Big Book of Everything bound in Genuine Human Hide. Those who read it gain immense knowledge and power from it, but it tends to disappear before the reader can read too much.
- The Brotherhood of Steel thinks they're this. It's more justified in Fallout, where they keep the secrets of advanced pre-War tech out of the hands of people who might misuse them, like Unity. In Fallout 2 it's less justified, as their status has diminished somewhat in the wake of the NCR's rise to power, where we see more of their tendency to raid towns, steal any tech more advanced then a lightbulb and kill anyone who resists, all in the name of "keeping advanced technology out of the wrong hands". By Fallout: New Vegas they're basically glorified raiders in power armor, and their zealous adherence to their antiquated dogmas and methods is slowly killing them.
- Fallout: New Vegas:
- The Good or Neutral Karma ending of Old World Blues sees the Courier becoming the new guardian of the Big Empty, releasing new and old technology to help the people of the Wasteland on an as-needed basis.
- The Courier also becomes this if they keep the secret of Archimedes II from the NCR and/or Legion.
- In League of Legends Nasus holds knowledge of the magic of life and death, although he does teach it to people who are judged worthy by his brother Renekton. The number of people who weren't worthy and wanted the power eventually drove Renekton insane and he started slaughtering first them, then everyone.
- Killer7: Christopher Mills knows that Garcian's the main personality, not Harman, and also knows that the government is controlled by the Secretary of Education. This gets him killed.
- The Secret World has a fairly benign example in the form of the mysterious half-magic half-mechanical Bees; native to the Hollow Earth, they not only grant the player character his/her magical powers at the start of the game, they also provide nuggets of ancient knowledge that they've been collecting over the eons in the form of the lore entries you track down. Of course, given that the only way anyone can access the Bees' "education protocol" is by actually being touched by the bees, the main price for all this forbidden knowledge involves having your entire life turned upside-down in much the same way the player character's was. The other price is actually having to track down each fragment of lore.
- Thief: The Keepers represent Neutrality between the chaos of the Pagans and the order of the Hammerites/Mechanists. They are an ancient order of scholars who have been recording all knowledge of the mystrious runes, that seem to have a great power over the city, but also include agents who are masters of stealth and spying. But as the third game reveals, even they have their own dirty secrets which are no less dark than those of the other two groups.
- In Star Control 3, the Captain somewhat becomes this. After learning the True Name of the Eternal Ones, he can share this knowledge with the Daktaklakpak, causing them to self-destruct.
- In Star Wars: The Old Republic, Darth Thanaton is the member of the Dark Council who controls the Sphere of Ancient Knowledge. That is, until the Sith Inquisitor defeats him and inherits his place on the Council.
- Stellaris has an entire faction of these, two if you buy the expansions. The first are the Keepers of Knowledge, an ancient and decrepit empire of scholars and hoarders who will occasionally reward you if you send them scientists (consenting or otherwise) and punish you if you research certain proscribed tech. The others are the Curator Enclaves, and they are far more friendly. They actively help you destroy pan-galactic threats and monsters, they send you scholars to help your research if you make the effort to befriend them, and offer a colossal 1/5th increase in research speed if you pay them a tithe.
- In Sarab, the Hubs are the only places science may be learned and they are very exclusive.
- Unsounded: The Inak have been forced from their homes and forced to "convert" to a religion that teaches that they are just soulless animals to be used by humans or be killed. Their elders, like Nana, keep their stories, religion and traditions secretly alive passing what they can on to the younger generations in pieces while keeping their true religion hidden from their human overlords.
- Avatar: The Last Airbender brings us Wan Shi Tong, He Who Knows Ten-Thousand Things, a spirit in the form of a giant owl who seeks to collect all information in the universe. He once had it all on display in a library for all to see, but eventually realised how humans used this information to hurt and kill others, and decided all knowledge should be forbidden.
- The Grim Adventures of Billy & Mandy: Parodied with a giant worm girl was forced to guard the "secrets of the universe". She then lured hundreds of kids into finding it, the secret being that there's no secret. When Mindy finds it, she has to take her place in guarding the secret while the worm goes free.